Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP third generation partnership project
BTS base transceiver station
C/I carrier-to-interference (—ratio)
CINR C/I
DL downlink
FDD frequency division duplex
HO handover
ID identification
PHY physical layer or L1
PI-type type of PLL with proportional and integral branches in the loop filter
PLL phase locked loop
QoS quality of service
RNC radio network controller
RSSI received signal strength indication
SNR signal-to-noise-ratio
SS subscriber station (fixed or mobile)
TDD time division duplex
UL uplink
UMTS universal mobile telecommunications system
UTRANUMTS terrestrial radio access network
This invention is related to synchronization within a network when a primary synchronization mechanism fails. Signaling among different base stations/cells of a network are synchronized to some common source or common control signal in order to facilitate re-use of radio resources (frequencies, spreading codes) in the uplink and downlink and to facilitate handovers of mobile subscriber stations (user equipment or mobile stations/terminals), among other reasons. This is done generally by an external timing reference (global positioning satellite clock signal; control signalling from a higher network node, etc.). The individual subscriber stations under control of the cell/base station BTS use timing references from the BTS to maintain their own synchronization to that cell. This may be through a synchronization channel, timing references in other control signals, and the like.
Any network that runs in TDD mode must be synchronized in order to avoid interference between UL & DL directions. Also any network (TDD or FDD) that supports soft macro diversity must be synchronized (macro BTSs are transmitting the same PHY signal or receiving the same PHY signal for later combining). If one or multiple BTSs lose their synchronization to the common timing reference, then the network performance will usually be first impaired and finally the out-of-sync-BS possibly needs to shut down all DL transmissions in order to avoid producing interference to the rest of the network. The BS UL receiver may still continue operating, but UL macro diversity will most likely be unusable if BTSs are not synchronized.
When an external timing reference of a base transceiver station BTS fail (e.g., failure of the source, transmission medium, or internal failure of the synchronization procedure within the BTS), the BTS can operate with its internal timing reference oscillator until the oscillator stability falls outside a predetermined threshold after which accuracy is no longer guaranteed. This internal oscillator may be used to approximate synchronization to the network/external source. But since the internal oscillator is no longer corrected to the external timing source it can be expected to drift over time, and so this reliability threshold is often given by a pre-calculated timer value that is computed based on expected drift (quality of the reference oscillator), temperature, and a few other factors. Generally, a BTS that continues operation using its internal oscillator timing after it has lost the external synchronization reference is said to be in a ‘holdover mode’.
A fairly serious problem exists when operating in the holdover mode, in that eventually a complete shutdown will be required once the pre-calculated timer value expires and the external timing source remains unavailable. This is because continued DL transmissions at least would tend to interfere with transmissions of other BTSs that may still be properly synchronized. So long as the BTS in question is synchronized to the external timing source or to its internal reference oscillator within the accuracy threshold, the BTS may continue to send DL transmissions. So extending the period during which a BTS may operate in the holdover mode addresses the problem of avoiding (or at least minimizing) BTS shutdown for lack of sufficiently accurate synchronization to the network. For example, if the holdover mode can be extended indefinitely, then the BTS would never need to shut down due to failure of primary synchronization mechanism.
The loss of synchronization has been addressed in the prior art by several different approaches. The prior art solutions typically take one of two approaches. The first approach uses a secondary external reference clock source (e.g. from GPS-based sync to E1-based sync feed). For TDD-mode, 3GPP specifies in TS 25.402 an over-the-air synchronization method whereby the BTS references specialized synchronization bursts and protocols. The second approach relies on determining the accuracy of the BTS's internal crystal oscillator. When the synchronization reference is lost, the voltage control of the internal oscillator is frozen to the current value. By examining the oscillator's short time stability & aging specifications and its sensitivity to external factors (like temperature, supply voltage stability, control voltage stability) a time limit is defined after which it can not be guaranteed that the BTS timing is good enough to avoid interfering with transmissions in other cells of the network. At this point the DL transmissions from the suspect BTS must be switched off.
The problem is that when an internal fault occurs within the suspect BTS whereby it can no longer synch to the functioning external common reference clock, a secondary external reference clock is likely to also be unusable for that suspect BTS. The other approaches are seen to potentially maximize the time by which the internal reference clock may be used for DL transmissions at least, but necessarily the internal oscillator will drift beyond the minimum guarantee of accuracy. In both instances the suspect BTS will need to be shutdown, at least for DL transmission, causing major disruptions in network access and coverage until repairs are made. What is needed is another backup synchronization method and apparatus to extend synchronization with the network beyond the accuracy of the internal oscillator.